In the manufacture of printed circuit boards for industrial use, frequently literally thousands of small diameter holes must be drilled into each circuit board so that appropriate electronic components may be mounted into the holes and soldered onto the boards. In volume production of the circuit boards, the drilling of holes is accomplished by computer controlled automatic drilling machines in which the printed circuit boards are usually mounted on a worktable movable in a horizontal X-Y plane. Usually, the worktable is moved horizontally in an incremental fashion beneath one or more drill spindles so that the pin holes may be drilled at appropriate drilling locations. Drilling of the circuit boards is accomplished by advancing each drill spindle downward through a vertical drilling stroke. This is a very rapid and satisfactory procedure for the mass production of printed circuit boards.
Since the diameter of the holes drilled in the printed circuit boards can be extremely small, for example on the order of the size of human hairs, it is imperative that the positioning of the holes at the appropriate drill locations or drill sites be exact and precisely controlled. This is particularly true where, as is often the case, components are subsequently mounted into the circuit boards automatically by machines which rely on predefined and precise placement and positioning of the holes.
During a normal drilling process, the worktable moves incrementally from drill site to drill site so that the spindle, which holds a drill tool, may drill a hole at each such predefined drill site before proceeding to the next drill site. As the worktable approaches each new predefined drill site and positions itself at the drill site, a computer control system delays initiation of the drill stroke for a predefined period of time. This delay allows the controller to further refine the position of the worktable relative to the drill spindle. Also, the delay results in a decrease in the velocity of the worktable. This period of delay is known as a "dwell time." The longer the dwell time, the more precisely the worktable can be positioned, thereby increasing the degree of hole placement accuracy.
An undesirable consequence of lengthy dwell times prior to each drilling operation is an increase in the time required to drill each hole and therefore a decrease in the efficiency or "throughput" of the drilling machine. Although the dwell time for any single hole is relatively short, each circuit board drilled usually requires the drilling of thousands of holes, for example as many as 20,000 or more holes per board. Consequently, when the dwell time for each hole is multiplied by the number of holes to be drilled in the board, a substantial increase in the overall time required to drill a single circuit board results.
In the prior art, a nominal dwell time is used for each drilling machine. Where a nominal dwell time is used, the chosen dwell time is normally longer than necessary for many drilling operations, thus ensuring that the worktable is given sufficient time to refine its position. Consequently, time is normally consumed in achieving a level of hole placement accuracy and stability that frequently is not warranted under the particular drilling conditions and requirements of that drilling operation. Over the course of drilling hundreds of thousands of holes, a substantial amount of time is unnecessarily wasted and the throughput of the drilling machine suffers significantly. Conversely, in some situations, the nominal dwell time chosen may not provide a sufficient level of hole placement accuracy for certain sensitive drilling operations and, in such a case, hole accuracy and thus product quality suffers unacceptably.
Thus, a method and apparatus is needed for optimizing the dwell times used during the drilling of printed circuit boards in order to improve the throughput of the drilling machine, and the quality and accuracy of the holes drilled.